fix(opencode): guard state parsing, lint the .tsx, correct an overstated doc claim

Three findings from a cross-branch review of the competing opencode
implementations, all of which are fair.

1. `readState` type-guards the parsed JSON now. A bare try/catch does not
   cover it: `JSON.parse('null')` succeeds and returns null, so the catch never
   fires and the next `state.project` throws a TypeError that takes the plugin
   down. Verified the crash before fixing; a test pins the guard in the embedded
   copies. Credit to the competing 'opencode-mine' branch, which had this right.

2. eslint now covers `src/opencode-ext/*.tsx`. The glob was `*.ts` only, so the
   300-line TUI plugin — the largest file in the addon — was linted by nothing.
   It was typechecked, which is why this went unnoticed. Confirmed the rules
   actually fire on it rather than the file being silently skipped. The
   'abhishek' branch was the only entry that got this right.

3. docs/opencode-extension.md overstated the security argument. "The token would
   sit in a 0644 opencode.json" is not a point against a `type: local` stdio
   bridge, which needs no token at all because it reads your own credentials.
   That reason is a consequence of having picked the HTTP gateway, not a
   justification for it. The docs now lead with the real reason — live
   re-pointing without a restart — and state the trade honestly.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BVwuCjuMoA13gmzYEfcrNP

eslint.config.js

@@ -3,7 +3,7 @@ import tsparser from '@typescript-eslint/parser';
 
 export default [
   {
-    files: ['src/*/src/**/*.ts', 'src/pi-ext/*.ts', 'src/opencode-ext/*.ts', 'src/prime-agent-ext/*.ts'],
+    files: ['src/*/src/**/*.ts', 'src/pi-ext/*.ts', 'src/opencode-ext/*.ts', 'src/opencode-ext/*.tsx', 'src/prime-agent-ext/*.ts'],
     languageOptions: {
       parser: tsparser,
       parserOptions: {